Traditional friction brakes and retarders do not recover any energy during braking. One example of a retarder is an eddy current retarder that absorbs energy for braking by dissipating the energy as heat.
In a parallel hybrid design for driving a drive axle, motors are used to recover braking energy. This braking energy is stored in batteries or ultra-capacitors, however, current designs are very inefficient. To improve efficiency for current systems, significant changes to a vehicle driveline would be required to increase motor/generator speed relative to driveline speed. This increase is especially important for low-speed vehicle applications such as mass transit and refuse collection applications.
As known, motor efficiency is highest when the motor is operating within a high speed range. As motor speed decreases, i.e. approaches zero, motor efficiency correspondingly decreases, i.e. approaches zero. For example, 3000 rpm is a typical motor operating speed and when a motor is operating at this level, motor efficiency is high. However, when the motor is operating at only 500 rpm, motor efficiency is drastically reduced. Low-speed, stop-and-go vehicle applications, typically operate within the 500 rpm range.
Thus, there is a need for a motor/generator combination for a parallel hybrid system that can operate in a high efficiency range even for low-speed applications.